North Korea celebrated the birth anniversary of late leader Kim Jong-Il Sunday after promoting key military officials including the chief of its rocket unit, state media said.

Kim Rak-Gyom, the commander of the Korean People's Army (KPA) Strategic Rocket Force Command, was promoted to colonel general at the order of leader Kim Jong-Un, the Korean Central News Agency (KCNA) reported late Saturday.

The unit is in charge of the country's mid- and long-range missiles programme.

Dozens of other senior military officials were also promoted to the rank of lieutenant general and major general, the KCNA said.

The latest round of military promotions -- often announced on key political anniversaries -- came as the North celebrated Jong-Il's birthday.

The country marks the birthdays of both late leaders -- Jong-Il's on February 16 and his father Kim Il-Sung's on April 15 -- as major national holidays.

Thousands of party and military officials convened in the capital Pyongyang to pledge loyalty to Jong-Un, television footage showed on Sunday.

"Everybody, let's rally around the party centre led by the great leader Kim Jong-Un and robustly fight... to complete the revolution!" the ceremonial head of state Kim Yong-Nam said in a speech at an indoor stadium.

The crowds stood and clapped in unison, chanting "Mansei (Long live)!" as Jong-Un sat stone-faced at a podium along with top officials including Choe Ryong-Hae, the director of the KPA's political bureau who is seen as the country's unofficial number two.

Jong-Un, as well as top army and party cadres, also visited the Kumsusan Palace of the Sun in the capital to pay tribute to the embalmed bodies of his father and grandfather, the KCNA said.

The military promotions come after an analysis of new satellite images posted on the 38 North website showed stepped up excavation activity at the North's main nuclear test site, although there were no signs that any further test was imminent.

A successful rocket launch staged by Pyongyang in December 2012 raised alarm over the isolated country's growing missile capability.

North Korea has tested missiles that could strike the South or Japan but has not yet tested a working inter-continental ballistic missile it claims to have.

The Kim family has ruled the communist state for more than six decades through a pervasive personality cult and an iron fist.

Jong-Il, who died of a heart attack in December 2011, was succeeded by his son Jong-Un.